This invention relates to the thermal processing and restoration of used asphalt paving materials after they have been removed from road surfaces by milling, grinding or ripping. After the bituminous paving materials have been removed from the roadbed, they are hereafter referred to as recycled asphalt pavement (“RAP”).
It is generally known that the majority of existing roadways, both concrete and bituminous asphalt, undergo constant repair and surface overlay with new hot mix asphalt to achieve and maintain safe and comfortable high speed riding surfaces.
In recent years, new equipment has been introduced to the road paving industry in the form of pavement milling or grinding machines. The science of preparing an old roadbed base for new resurfacing is now commonly referred to as milling. Both state and Federal Department of Transportation (DOT) agencies throughout the country have readily accepted the science of milling.
The milling of old road surfaces provides a number of advantages in preparing the old roadbed for resurfacing. Milling not only ensures a new, smooth and level base for the new hot mix overlay, but at the same time lowers the road bed height to maintain bridge deck clearances and curb and gutter depths. Grinding or milling is also beneficial in removing potholes, old cracks, joint seams, and ruts along with other surface damage that would quickly reappear in a new surface overlay if not repaired. With many of the state and Federal DOT agencies now requiring the milling of road surfaces before permitting new overlay, there is an increasing inventory of asphalt millings being generated. The piles of discarded asphalt millings are becoming problems for land use, aesthetics and the environment. Attempts at reuse have proven difficult.
Generally speaking, asphalt plants average 400 tons per hour to 600 tons per hour production ranges and 15% to 30% RAP can be injected into these plants. When attempting to inject cold wet RAP into hot mix plants, the existing processes rely almost entirely on super heated virgin aggregates (600° F. to 900° F.) to conductively transfer enough heat to the cold wet RAP for drying and heating all materials to a mixing temperature of 300° F. The sudden and violent steam expansion that is created when the super hot aggregate (600° F.-900° F.) encounters the cold wet RAP instantly overloads exhaust system airflow capacity.
If the RAP injection is being done in a “drum mixer” type asphalt plant where all injection of RAP is done inside of the aggregate dryer, the steam explosion restricts the dryer air flow and overloads the exhaust vacuum system forcing the operator to lower plant production rates to restore exhaust vacuum and air flow on the drum to maintain final mix temperature.
When RAP injection is attempted on “batch” type asphalt plants, the cold wet RAP is injected into the weigh hopper section of the batcher above the pug mill mixer and when the RAP instantly mixes with the hot aggregate in the weigh hopper, a violent steam explosion occurs blowing steam and dust into the air creating fugitive emissions and sometimes even damaging the hopper section from pressure surges. Injecting RAP into batch plants can also be very restrictive in tonnage output so as to avoid damaging the plant. Generally, batch plants cannot accept more than 15% to 20% RAP recycling.
In either methods of recycling RAP, whether in a drum mixer or in a batch plant, the pre-drying and super heating of the virgin aggregates is the only method of heat transfer to the RAP. The RAP must be dried and then heated to the mix temperature via conduction only from the aggregate. The virgin aggregate must be super heated in order to load enough heat in the material to transfer the energy to the RAP but still retain enough heat to have all material exit at 300° F. By having to elevate aggregates to 700, 800 and 900° F., the rotary dryers that heat the sand and stone must be subjected to extreme temperatures and this is causing many premature failures. Dryers are manufactured to operate with continuous skin temperatures in the 500° F. range and less. When these drum shells are exposed to the higher temperatures required running RAP mixes, they will crack and fail as well as experience extreme and premature wear.
Therefore, the following negative aspects of current methods for processing RAP into hot mix asphalt in plants need to be addressed: (1) Dryers must heat virgin aggregates to excessively high temperatures to dry and heat RAP and can therefore inflict heat damage, premature wear, and failure to the process, (2) Plant productive capacity drops off dramatically when running RAP due to exhaust system and dryer burner overloads from RAP steam blockages within dryers, (3) Batch Plants are limited as to RAP injection capacity due to fugitive emissions problems and potential plant damage due to violent steam explosions, and (4) RAP injection percentages are limited by aggregate temperatures for conductive heating. Heating aggregates above 600° F. can cause aggregate to fracture and allow mix gradations to drift out of specifications.
Needs exist for new recycling methods and apparatus for the recycling of asphalt pavement millings.